Some Insights on Coffee Leaf Rust (Hemileia vastatrix)

By Emma Sage, SCAA Coffee Science Manager

It is clear that many Central American countries are experiencing a devastating outbreak of coffee leaf rust, which is affecting current yields and will likely affect subsequent harvests. The SCAA is committed to utilizing association resources to address this problem with a variety of actions. Part of that commitment is helping those of us in consuming countries understand what is going on in some of our most important specialty coffee producing countries.

What is coffee rust?

The Coffee Rust is an obligate parasitic fungus, which means it is a microorganism that must take energy and nutrients from a specific live host (coffee) and reproduces differently than either plants or animals. Coffea arabica is the most susceptible species to this fungus, but C. canephora (Robusta) can also be affected. The fungal life cycle is a complex and ingenious one, where organisms asexually produce thousands of tiny spores (reproductive bodies) that can travel in water, rain, or air and remain viable for long distances (Kushalappa and Eskes 1989; Gouveia and others 2005). Once a spore lands on a leaf, it can sit until conditions are right. At that point, it germinates and enters the leaves through stomata, the pore-like openings utilized for plant gas-exchange (Muller and others 2009). The coffee rust is an obligate parasite to coffee, meaning that it must find a coffee host in order to complete its life cycle. Most scientists believe that it once had or still has alternative host, but one is yet to be identified. Despite the importance of this destructive organism there is still much to be learned on the biology of coffee rust.

Photo by The American Phytopathological Society

Even though coffee rust is found everywhere specialty coffee is grown, it thrives under a specific set of climatic and environmental conditions. Free moisture (rain or heavy dew) is perhaps the most important factor that influences the success of coffee rust (Nutman and others 1963). Water is necessary for spore germination and can also act to disperse spores. Periods of rain are usually when outbreaks occur, which is a big reason rainfall patterns are so important.  Temperature also effects rust development. If it is too cold (below 15°C) spores will not germinate and the growth of the fungus will be slowed. Likewise, if it is too warm (more than 35°C) the fungal growth with slow. The most optimal temperature for the growth and proliferation of the rust is between 21-25°C (Nutman et al. 1963). Light can also change how the fungus affects coffee plants, although there is some mixed evidence around this. Leaves under full sun (high intensity light) have been shown to be more susceptible to rust, and once infected light can alter the growth rate of the fungus. However, very high intensity light has also been shown to slow the fungus growth (Muller et al. 2009). Typically, a single spot of rust on a leaf can produce 4-6 generations of spores over a 3-5 month period. This could result in the release of hundreds of thousands of individual mature spores, ready to start the process all over again.

The environment that coffee is cultivated within is the primary determinant of the moisture, temperature, and sunlight intensity. There has been some debate over which farming strategy is best to prevent rust outbreaks, i.e. whether shade growing or full sun plantation style growing serves best. The spatial arrangement (planting density) of coffee trees and overall productivity of those plants can also influence the rust colonization of a plantation (Monaco 1977). On one hand, a heavy shade environment may buffer temperature extremes and increase the moisture retention of a system and therefore be conducive to rust colonization (López-Bravo and others 2012). On another hand, it is known that fruit load is correlated with outbreak severity , so under shade conditions, when fruit load is smaller relative to full sun, outbreaks can appear to be buffered (López-Bravo et al. 2012; Avelino and others 2006).

What does it do to coffee?

When the fungus first colonizes on a coffee tree, it presents with a slight discoloration on the underside of leaves. These spots quickly progress to yellow then an orange ‘dust’, which are the mature spores. Most commonly rust causes infected leaves to fall off, leaving the tree without leaves and at a considerable disadvantage for the rest of the fruiting season.  In severe cases the fungus can present on young leaf buds or fruits. Leaf rust usually does not result in plant death, but it is possible under extreme colonization.

A coffee plant can lose a substantial amount of foliage when attacked by coffee rust. When a coffee plant does not have the optimal amount of leaf area, it does not have the ability to accumulate adequate energy via photosynthesis and store up the appropriate resources for fruit production. This is why there is generally a loss of yield even the year after rust outbreaks (Avelino and others 2004).

Where did it come from?

Coffee rust has likely been around since arabica coffee was only growing wild in Africa, but was not ‘officially’ detected there until the 1870’s. Its first recorded impact began in the 1850’s, when a large outbreak in Ceylon (now Sri Lanka) devastated the coffee industry on that small island, ending in the crop being replaced with tea (Abbay 1876). By the 1960s it had spread throughout Indonesia, and in the 1970s it finally made its way to the Americas, to the Bahia State in Brazil (Correspondent 1970; Monaco 1977). From Brazil, despite the best efforts to quarantine or eradicate the pathogen, it was spread around central and South America over a 15 year period. From what is now known about the viability of spores, it is thought that inter-continental dispersal was possible through wind. Now, it is always around, just in varying degrees. Small outbreaks are typically controlled by fungicide management.

Why is it so bad now?

It is no secret that climate changes are already being felt by coffee growers in Central America and throughout the world. Changes in both temperature and rainfall likely contributed to this unusual outbreak. Warming temperatures at higher altitudes and the resulting shifts in moisture accumulation are likely allowing the rust to thrive in areas previously uninhabitable (Avelino et al. 2006). The case of leaf rust is an example of how changing patterns of, rather than the total annual amount of precipitation, can really alter an ecosystem. Disruptions in the typical dry season can create an environment more habitable for rust outbreaks. Smaller and more frequent rainfall in the region likely contributed to the perfect fungal habitat.

There are other factors that may have exacerbated this particular outbreak. Some are convinced that the changes in land use and coffee farming practices is the primary reason for recent outbreaks. They believe that the shift to high density monoculture allows the easier transmission of rust. What is really going on? We do not have all the facts. The exact perfect storm for a massive multi-country severe rust outbreak is currently an unfortunate puzzle. The truth is that since there is not a full understanding of the biology of this fungus it is impossible to understand what exactly could affect its growth and proliferation, let alone how to eradicate it. This seems to be another example of where the scientific research on coffee is sadly lagging behind that of other crops.

What can we do?

There is no quick solution to this problem. The outbreaks have already occurred, and are known to proliferate into multi-year events. The usual means of keeping the fungus at bay are no longer solutions to this problem. Since coffee rust spread over the producing regions of the world, fungicides have been used to lessen the outbreaks. Copper based fungicides are most common, but have short periods of effectiveness and applications must be timed carefully. In the end, these metallic fungicides can be detrimental to the environment and are limited in capacity to combat coffee rust.

There has been some research into a more biological or organic approach to combat the rust. Certain ‘hyperparasitic’ fungi (that are parasitic to parasites) have been identified in nature that prey on the coffee rust fungus (Muller et al. 2009). One such fungus, known as the ‘white halo’ fungus, has been getting lots of press lately. Scientifically, there is some evidence that this and possibly other microorganisms can reduce the viability of the rust (Jackson and others 2012; Vandermeer and others 2010; Haddad and others 2009). However, there has never been any practical application suggested or implemented for this biological control in coffee. Unfortunately, at this juncture there is no real way to implement these additional microorganisms on a large agricultural scale. Nor has their ability been proven to eradicate large rust outbreaks, at the size and severity that the industry is currently faced with.

In the longer term, it will become critical to develop high quality resistant varieties. Kent was perhaps the first variety to show good resistance to coffee rust in India. However, this resistance was lost after about 10 years of exposure (Rodrigues and Eskes 2009). This phenomenon of gradually losing resistance has also been noticed in some C. iberica and C. canephora varieties. This led to the discovery that there were many ‘races’ of coffee rust and that new races could develop (Muller et al. 2009). Today, there are over 45 races identified, although a select few are likely responsible for most outbreaks (Fernandez and others 2012; Rodrigues and Eskes 2009).  Currently, Catimors and Icatu show partial resistance to the rust, but lack many of the taste attributes desired by specialty coffee buyers. Unfortunately, the mechanism for resistance in coffee is not fully understood (Silva and others 2002; Silva and others 2008; Guerra-Guimares and others 2009). More research on how plants control the fungus will be necessary as the scientific community moves forward with identifying the specific genes that contribute to rust resistance.


To address this critical international problem World Coffee Research and regional cooperative Promecafe, with support from the Specialty Coffee Association of America and others, will be hosting an emergency summit on coffee rust this coming April. The purpose of this meeting will be the vetting and aligning of national coffee rust research and mitigation strategies. The summit will bring together relevant scientists, producer groups, NGOs, finance organizations, plant protection companies, and the coffee industry’s export and roasting sectors.


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